A superabsorbent polymer, or material, in general refers to a water-swellable, water-insoluble polymer, or material, capable of absorbing at least about 10 times its weight, and up to about 30 times or more its weight in an aqueous solution containing 0.9 weight percent sodium chloride solution in water. Examples of superabsorbent polymer may include a crosslinked partially neutralized polymer, including crosslinked polyacrylic acids or crosslinked starch-acrylic acid graft polymers, that are capable of absorbing large amounts of aqueous liquids and body fluids, such as urine or blood, with swelling and the formation of superabsorbent hydrogel, and of retaining the aqueous liquids under a certain pressure in accordance with the general definition of superabsorbent polymer.
The superabsorbent polymer hydrogel may be formed into particles, generally referred to as particulate superabsorbent polymer, wherein the particulate superabsorbent polymer may be post-treated with surface crosslinking, surface treatment, and other surface treatment to form particulate superabsorbent polymer compositions. The acronym SAP may be used in place of superabsorbent polymer, superabsorbent polymer composition, particulate superabsorbent polymer compositions, or variations thereof. A primary use of superabsorbent polymer and superabsorbent polymer compositions is in sanitary articles, such as babies' diapers, incontinence products, or sanitary towels. A comprehensive survey of superabsorbent polymers, and their use and manufacture, is given in F. L. Buchholz and A. T. Graham (editors) in “Modern Superabsorbent Polymer Technology,” Wiley-VCR, New York, 1998.
Sanitary articles, such as diapers, generally include an absorbent core that includes about 30-50% of cellulose fiber and about 50-70% of particulate superabsorbent polymer composition. It is a goal of future sanitary articles to make them smaller and thinner, for fit, comfort and aesthetic reasons and from environmental aspects. One way to accomplish this goal is to reduce the amount of fiber material and increase the amount of particulate superabsorbent polymer composition, wherein there may be less than about 30%, or less than about 20%, or less than about 10% of fiber material in the absorbent core. The particulate superabsorbent polymer composition of these next generation diaper constructions must have a sufficiently high stability and permeability in the swollen state, so that liquid can be transported through the swollen gel. In addition, the components of the sanitary articles must be compatible for the user wherein the components must have properties such as pH compatible with baby's skin, which has a pH of about 7.
Superabsorbent polymers may be prepared by initially neutralizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g., sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like in the presence of a caustic treatment, such as sodium hydroxide, and then polymerizing the product with a relatively small amounts of an internal, or monomer, crosslinker such as a di- or poly-functional monomers. The di- or poly-functional monomer materials may serve as covalent internal crosslinking agents to lightly crosslink the polymer chains, thereby rendering them water-insoluble, yet water-swellable. These lightly crosslinked superabsorbent polymers contain a multiplicity of carboxyl groups attached to the polymer backbone. These carboxyl groups generate an osmotic driving force for the absorption of body fluids by the crosslinked polymer network. The particulate superabsorbent polymer may be surface treated with surface crosslinking and surface treatment to enhance the properties of the particulate superabsorbent polymer.
Superabsorbent polymers and particulate superabsorbent polymer compositions, useful as absorbents in absorbent articles such as disposable diapers, need to have adequately high sorption capacity, as well as adequately high gel strength. Sorption capacity needs to be sufficiently high to enable the absorbent polymer to absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article. Gel strength relates to the tendency of the swollen polymer particles to resist deformation under an applied stress, and needs to be such that the particles do not deform under pressure, and fill the capillary void spaces in the absorbent member, or article, to an unacceptable degree, which is generally called “gel blocking”, thereby inhibiting the rate of fluid uptake, or the fluid distribution, by the member or article. Once gel-blocking occurs, it can substantially impede the distribution of fluids to relatively dry zones or regions in the absorbent article, and leakage from the absorbent article can take place well before the particles of absorbent polymer in the absorbent article are fully saturated, or before the fluid can diffuse or wick past the “blocking”
Permeability is a measure of the effective connectedness of a porous structure, be it a mat of fiber of a slab of foam or, in the case of this application, particulate superabsorbent polymer and particulate superabsorbent polymer composition, generally referred to as particulate superabsorbent polymer compositions herein, or SAP, and may be specified in terms of the void fraction and extent of connectedness of the particulate superabsorbent polymer compositions. Gel permeability is a property of the mass of particulate superabsorbent polymer compositions as a whole and is related to particle size distribution, particle shape, the connectedness of the open pores, shear modulus and surface modification of the swollen gel. In practical terms, the permeability of the particulate superabsorbent polymer composition is a measure of how rapidly liquid flows through the mass of swollen particles. Low permeability indicates that liquid cannot flow readily through the particulate superabsorbent polymer compositions, which is generally referred to gel blocking, and that any forced flow of liquid (such as a second application of urine during use of the diaper) must take an alternate path (e.g., diaper leakage).
Surface treatment of particulate superabsorbent polymers is already well-known. To improve the permeability of particulate superabsorbent polymers, ionic complexing of the carboxyl groups near the surface using polyvalent metal cations has been disclosed in prior arts. U.S. Pat. No. 6,620,889 discloses superabsorbents which are surface crosslinked with a combination of a polyol and a cation salt in aqueous solution. The salt's anion may be chloride, bromide, sulphate, carbonate, nitrate, phosphate, acetate or lactate. The use of aluminium sulfate as surface treatment for particulate superabsorbent polymer compositions is disclosed in reference WO 2005/108 472 A1, which discloses a process that includes treating a base polymer with a water-soluble multivalent metal salt and an organic acid or its salt. The multivalent metal salt is preferably aluminium sulfate. The organic acid or salt is selected from a range of acids that includes citric acid, glyoxylic acid, glutaric acid, succinic acid, tartaric acid and lactic acid, or alkali or ammonium salts thereof.
However, it is also known that aluminum sulfate is acidic, with a pH value of less than 4, and is well below the pH values of about 7 of human skin. Aluminum sulfate applied on the surface of particulate superabsorbent polymer will generate an acidic surface. Since a sanitary article comprising superabsorbent polymers is in contact with or is otherwise near or next to a user's skin, it is desirable to control the surface pH of superabsorbent polymers in order to reduce the risk of skin irritation.
It is also known that the solubility of aluminum ions and salts thereof such as aluminum sulfate, in water is pH dependent. At pH levels between 4 and 9.5, aluminum sulfate becomes insoluble in water and precipitation occurs, resulting in slurry of aluminum hydroxide. It is known that a slurry is more difficult to handle, than a solution, in a production process.
U.S. Pat. No. 5,559,263 discloses a method for the preparation of an aqueous aluminum solution having a pH between about 5.0 and about 9.0 at a concentration of at least about 3.1 percent by weight of aluminum. The solution comprises citrates as chelating ligands for aluminum ions. The chelating ligands may compete with the carboxyl groups near the surface of particulate superabsorbent polymer to form ionic complexes with aluminium ions, which may diminish the permeability enhancing effects of aluminium ions.
It is therefore an object of the present invention to provide a particulate superabsorbent polymer composition having improved compatibility with human skin. It is also an object of the present invention to provide a particulate superabsorbent polymer composition that exhibits excellent properties such as the ability to maintain high liquid permeability and liquid retention even when the amount of the particulate superabsorbent polymer composition is increased in percent by weight based on the absorbent structure.